1. Technical Field
This invention relates to using core sand for precision molding of metal castings, particularly aluminum, and more particularly to enhancement of metal yield, metal properties and quality features such as surface finish using such casting technique.
2. Discussion of the Prior Art
Precision-type sand casting (using core type sand such as zircon or silica) is known and has been used for at least 50 years in the commercial production of automotive castings, such as cylinder heads and blocks. This technique has many advantages, but it leaves certain features to be desired, such as increasing yield and improving the microstructure or surface finish of the casting, and increasing the speed of producing castings by such technique.
Risers, and to a lesser extent venting, have regularly been required in the molding system when sand casting aluminum. This is mandated to avoid shrinkage and pin holes in the solidifying regions. The risers serve as a molten reserve of aluminum that stays hotter to feed such regions. Unfortunately such risers adversely affect yield of the process.
Such sand casting processes usually rely on gravity to feed molten metal to a runner system with the pressure head from the metal filling the sprue serving to provide a low level of pressurization for the metal in the runners. Due to the need to fill the risers during the pour, the cycle is slowed, allowing the molten temperature to drop and reach adverse temperature levels, particularly near the end of the mold filling. Thus, it is traditional to pour at higher metal temperatures to compensate for this aspect. This results in (i) a poorer surface finish, (ii) a poorer microstructure in the last metal to solidify, and (iii) poor production cycling.
The gravity runner system typically has abrupt changes in direction of sections of the runner system; again, the metal must be poured at higher temperatures to maintain good fluidity over the slower cycle of the casting pour; this results in a flow that is somewhat turbulent. Heat is readily transferred to the sand walls of the mold, often causing the sand particles to fracture, leading to poor surface finish for the metal casting. The higher pouring temperature tends to produce poorer metal microstructure in the regions last to solidify, producing a microstructure with wider dendritic arm spacing than desired.